(1) Field of the Invention
The present invention relates generally to organic electro-luminescence devices (OLEDs) and particularly to a top-emission organic electro-luminescence device.
(2) Description of the Prior Art
Organic electro-luminescence devices become popular in display technology these days. Comparing to another popular display—liquid crystal display (LCD), the organic electro-luminescence devices have the advantage of self light-emitting. Hence, backlight module is no more needed in a panel display applied OLEDs.
Please refer to FIG. 1, which is a cross-section view of the structure of an organic electro-luminescence device. The electro-luminescence device 100 comprises, in the sequence from bottom to top, a substrate 102, a hole injection layer 104, a hole transport layer 110, a light-emitting layer 120, an electron transporting layer 140, and an electron injection layer 150. Wherein the hole transport layer 110, the light-emitting layer 120, and the electron transporting layer 140 are made of organic materials and fabricated via organic processes.
Combination of electron and hole in the light-emitting layer 120 would release photons. In the process, electrical energy is converted to optical energy. As shown in FIG. 1, the generated light leaves the electro-luminescence device 100 through its bottom (the substrate 102). So the electro-luminescence device 100 is also named as an “bottom-emission type”. Wherein, the hole injection layer 104 is usually made of ITO material, which is know as a light-transparent material. The electron injection layer 150 is made of light-reflecting metal materials with low work function, for instance, Al, Ca, Mg, and Ag are well suited. The light-reflecting character of above materials forces the generated light leave the organic electro-luminescence device 100 via the substrate 102.
However, some essential wires are disposed on the substrate 102. As to an active-matrix organic light emitting display (AM OLED), it even needs thin film transistors (TFTs) array on the substrate 102. Said wires or TFTs would restrict the light utilizing efficiency because of a limited aperture ratio.
Please refer to FIG. 2. A prior “top-emission type” organic electro-luminescence device 100a is presented. Wherein, the generated light leaves the organic electro-luminescence device 100a via the electron injection layer 150. In this case, the electron injection layer 150 should be made of light transparent materials, usually ITO, which is also conductive. Because the needed wires or TFTs are made on the substrate 102, no aperture ratio issue should be concerned in the top-emission type organic electro-luminescence device 100a. Comparing to the organic electro-luminescence device 100 of FIG. 1, the organic electro-luminescence device 100a could provide a better light utilizing efficiency.
While applying conductive transparent material like ITO for the electron injection layer 150, a major issue is that sputtering process of the ITO material would damage the organic materials (140) beneath. Please refer to FIG. 2, a buffer layer 145 made of CuPc is therefore provided by G. Parthasaraty et al. Before forming the electron injection layer 150, the buffer 145 is formed firstly to withstand the sputtering process. However, while practical estimated, the electron injection efficiency of the organic electro-luminescence device 100a is too poor to apply in commercial application. An experiment data shows that the organic electro-luminescence device 100a according to G. Parthasaraty et al. has an electron injection efficiency of 3.7 mA/cm2 with a working voltage at 6 V, mean while, the energy efficiency is less than 1 lm/W.
Please refer to FIG. 3. Another prior organic electro-luminescence device 100b provided by Hung Liang-sun et al. is presented. Before sputtering the buffer layer 145, a LiF/AL interface layer 143 [thickness of AL is about 10˜20 A], which is a thin metal layer, is sputtered firstly. Wherein, the buffer layer 145 still comprises CuPc compound. Diffusion of Li from the LiF/AL interface layer 143 to the underneath organic materials (140) could lower the energy barrier of Al chelate in the electron transporting layer 140. From this view, Hung Liang-sun et al. seems have been solved the problem of the organic electro-luminescence device 100a taught by G. Parthasaraty et al.
However, please still refer to FIG. 3, Li of the LiF/AL interface layer 143 could further diffuse into the light-emitting layer 120. This may decrease the life time of the organic electro-luminescence device. Furthermore, the light-emitting layer 120, the electron transporting layer 140, and the buffer layer 145 is fabricated in organic chamber, but the LiF/AL interface layer 143 is fabricated in a metal chamber. Hence the fabrication process of the organic electro-luminescence device 100b comprises moving to the metal chamber after forming the light-emitting layer 120 and the electron transporting layer 140 in a organic chamber, then moving to a organic chamber, finally moving to a sputtering chamber to from the electron injection layer 150. The complicated process (at least three times of movement between different chambers) is a disadvantage of organic electro-luminescence device 100b. 
Therefore, how to provide a top-emission organic electro-luminescence device, which overcomes the mentioned problems—the damage of sputtering process, the low electron injection efficiency, the decreased life time of organic materials and the complicated fabrication process, is the major issue of the present invention.